Ag alloy of high discoloration resistance
BACKGROUND OF THE INVENTION 
     The present invention relates to Ag alloys of high discoloration resistance, and more particularly relates to improvement in color maintenance of Ag alloys generally used for building parts, interior decorations, kitchen utensils and silverware. 
     Au-Ag-Pd type alloys are generally known as typical As alloys of high discoloration resistance. Japanese Patent Opening Sho. No, 53-43620 also discloses another Ag alloy of white color, high corrosion resistance and excellent for machining. The alloy is suited for use for watchcases and contains Ag, Pd, Sn and Zn. Optionally, Mg, Al, Ge, In and Ni are added individually or in combination. In either of the two conventional Ag alloys of high discoloration resistance, it is essential to contain 10 or more % by weight of Pd for sufficient xanthation resitance. 
     Despite the relatively improved discoloration resistance, such conventional Ag alloys are very exepensive due to high content of costly Pd. In addition, high content of Pd provides the products with relatively blck tint, thereby marring the inherently beautiful color of Ag. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the primary object of the present invention to provide Ag alloy of low price and high discoloration resistance. 
     In accordance with the basic aspect of the present invention, Ag alloys comprise 0.2 to 9.0% by weight of In and 0.02 to 2.0% by weight of Al. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As stated above, Ag alloys in accordance with the present invention comprise 0.2 to 9.0% by weight of In and 0.02 to 2.0% by weight of Al. No improvement in xanthation resistance is expected when the content of In falls below 0.2% by weight, whereas the inherent beautiful color of Ag is degraded when the content of In exceeds 9.0% by weight. Any weight percent content of Al below 0.02 would enable improvement in discoloration resistance. Chlorination resistance of the product is much degraded when weight percent content of Al exceeds 2.0% by weight. As well known, addition of In raises discoloration resistance of Ag. However, sole addition of In more that 10% by weight adds yellow tint to the product, and such yellow tint is much furthered by xanthation. Addition of Al well oppresses yellow discoloration caused by addition of In and naturally reduces percent cconten of In, thereby raising xanthation resistance of the product. No improvement in xanthation resistance is expected by sole addition of Al. 
     In one preferred embodiment of the present invention, Ag alloys further comprise 0.3 to 3.0% by weight of Cu for improvement in mechanical properties, more specifically hardness of the product. No appreciable effect is observed when the content is below 0.3% by weight whereas any percent content above 3.0% by weight would degrade xanthation resistance of the product, admittedly increasing the hardness. 
     In another preferred embodiment of the present invention, Ag alloys further comprises Cd, Sn, Ga and Zn individually or in combination for improvement in xanthation resistance and suitability for casting. 
     With the above-proposed composition, elements forming the Ag alloys are believed to form an inert film on the surface of the product, which makes the product well resistant against xanthation and chlorination, thereby accordingly raising discoloration resistance. 
    
    
     EXAMPLES 
     Samples Nos. 1 to 34 having compositions shown in Table 1 were prepared. The surface of each Sample was polished for evaluation of the tint. Next, the Sample was immersed for 10 hours in a Na z  S bath of 0.1% concentration and in NaCl bath of 5% concentration, respectively, for investigation of degree of discoloration. The results are shown in Table 2 in which X indicates high degree of discoloration, Δ indicates some degree of discoloration and O indicates substantially no discoloration. Samples Nos. 33 and 34 were prepared merely for comparison purposes. 
     
                       TABLE 1                                                     
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Sample Composition in % by weight                                         
No.    In     Al      Cu   Cd    Sn   Ga   Zn   Ag                        
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1      0.1    0.01                              Bal                       
2      0.2    0.02                              Bal                       
3      2.0    2.0                               Bal                       
4      4.0    2.0                               Bal                       
5      6.0    1.5                               Bal                       
6      9.0    1.5                               Bal                       
7      9.0    0.02                              Bal                       
8      10.0   4.0                               Bal                       
9      6.0    2.0     0.23                      Bal                       
10     6.0    1.0     1.5                       Bal                       
11     6.0    1.5     3.0                       Bal                       
12     7.0    1.5     4.0                       Bal                       
13     8.0    1.3     1.8        1.0       1.5  Bal                       
14     7.0    1.0     1.15            1.0  1.7  Bal                       
15     8.0    1.0     2.0  1.6        3.0       Bal                       
16     8.0    1.0     3.8  0.75  0.85 0.7       Bal                       
17     5.0    1.0     1.0  0.2   0.7  0.5  1.0  Bal                       
18     6.0    1.0          3.0                  Bal                       
19     5.0    1.0                3.5            Bal                       
20     6.0    0.03                         0.01 Bal                       
21     6.0    1.0                          4.0  Bal                       
22     4.0    1.0                          7.0  Bal                       
23     6.0    0.03         0.01       0.01      Bal                       
24     7.0    0.8                1.5       2.0  Bal                       
25     4.0    1.0                     4.5  3.0  Bal                       
26     4.0    0.3          0.3   0.5  0.5       Bal                       
27     10.0   0.3          1.0   1.9  1.45 2.1  Bal                       
28     4.5    0.01                    0.01 0.01 Bal                       
29     3.5    0.8          0.7   0.5       0.5  Bal                       
30     6.5    4.0                0.4  0.8       Bal                       
31     3.0    0.8          0.5   0.2  1.0  0.9  Bal                       
32     3.0    1.0          1.8   2.5  1.3  2.0  Bal                       
33     5Au--25Pd--Ag alloy                                                
34     100% Ag                                                            
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 Bal: in balance                                                          
 
    
     
                       TABLE 2                                                     
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Sample  Degree of discoloration                                           
No.     0.1% Na.sub.2 S                                                   
                    5% NaCl   Tint                                        
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 1      Δ     O         Silver                                      
 2      O           O         Silver                                      
 3      O           O         Silver                                      
 4      O           O         Silver                                      
 5      O           O         Silver                                      
 6      O           O         Silver                                      
 7      O           O         Silver yellow                               
 8      Δ     Δ   Silver yellow                               
 9      O           O         Silver                                      
10      O           O         Silver                                      
11      O           O         Silver                                      
12      Δ     O         Silver                                      
13      O           O         Silver                                      
14      O           O         Silver                                      
15      O           O         Silver                                      
16      Δ     O         Silver                                      
17      O           O         Silver                                      
18      O           O         Silver                                      
19      O           O         Silver                                      
20      O           O         Silver                                      
21      O           O         Silver                                      
22      O           Δ   Silver                                      
23      O           O         Silver                                      
24      O           O         Silver                                      
25      O           Δ   Silver                                      
26      O           O         Silver                                      
27      O           O         Silver                                      
28      Δ     O         Silver                                      
29      O           O         Silver                                      
30      O           Δ   Silver                                      
31      O           O         Silver                                      
32      Δ     Δ   Silver                                      
33      O           O         Metallic black                              
34      X           O         Silver                                      
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     It is clear form Table 2 that content of In below 0.2% by weight assures no good discoloration resistance against Na 2  S. When the content of In exceeds 9% by weight the product assumes yellow tint quite different form the inherently beautiful color of Ag. Percent content of Al above 2.0% by weight assures no good discoloration resistance against NaCl. When content of Cu exceeds 3.0% by weight, the product exhibits no good discoloration resistance against Na 2  S. Contents of Cd, Sn, Ga and/or Zn beyond 6.5% by weight rather degrades discoloration resistance and makes the product brittle due to formation of inter metallic compounds. 
     Samples Nos. 35 to 43 as shown in Table 3 were prepared for measurement of mechanical properties and the results of are shown in Table 4. Here sample 41 is the same in composition as Sample 13, Sample 42 is the same as Sample 14 and Sample 43 is the same as Sample 15 in Table 1, respectively. 
     
                       TABLE 3                                                     
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Sample Composition in % by weight                                         
No.    In     Al      Cu   Cd    Sn   Ga   Zn  Ag                         
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35     4.0    2.0                              Bal                        
36     4.0    2.0     0.3                      Bal                        
37     6.0    2.0     0.5                      Bal                        
38     8.0    1.0     3.0                      Bal                        
39     7.0    1.5     2.0                      Bal                        
40     7.0    1.5     3.0                      Bal                        
41     8.0    1.3     1.8        1.0       1.5 Bal                        
42     7.0    1.0     1.15            1.0  1.7 Bal                        
43     8.0    1.0     2.0  1.6        3.0      Bal                        
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                       TABLE 4                                                     
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Sample       Mechanical properties                                        
No.          Elongation in %                                              
                          Hardness                                        
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35           43           75                                              
36           42           80                                              
37           38           93                                              
38           35           127                                             
39           36           125                                             
40           31           140                                             
41           29           145                                             
42           35           123                                             
43           30           138                                             
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     It is clear form the results shown in Table 4 that addition of Cu causes moderate increase in hardness. Although ductility of the product is somewhat degraded, the product is still acceptable for working. Any percent content of Cu over 3.0% by weight, however, would cause unacceptable lowering in ductility and, in addition, mar discoloration resistance. 
     Sample 3 was immersed in a na 2  S bath of 0.1 concentration for 10 hours after heat treatment at various temperatures for various periods and degrees of discoloration were measured. The heating periods are shown in Table 5 with the results of measurement. In Table 5, O indicates substantially no discoloration, Δ indicates discoloration and X indicates solution of the sample. 
     As is clear from the data in Table 5, heating at a temperature below 220° C. would cause no appreciable improvement in discoloration resistance whereas the sample melts beyond 900° C. Further, it was confirmed that no appreciable effect can be observed when the period is shorter than 1 min. Measurement was carried out using the above-described Samples and the same result was obtained in the compositions as set out in the appended claims. 
     
                       TABLE 5                                                     
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Temperature                                                               
          Period in min.                                                  
in °C.                                                             
          0.5   1.0    30  60   120  240   480  960                       
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150       Δ                                                         
                Δ                                                   
                       Δ                                            
                           Δ                                        
                                Δ                                   
                                     Δ                              
                                           Δ                        
                                                Δ                   
200       Δ                                                         
                Δ                                                   
                       Δ                                            
                           Δ                                        
                                Δ                                   
                                     Δ                              
                                           Δ                        
                                                Δ                   
220       Δ                                                         
                O      O   O    O    O     O    O                         
300       Δ                                                         
                O      O   O    O    O     O    O                         
350       Δ                                                         
                O      O   O    O    O     O    O                         
400       Δ                                                         
                O      O   O    O    O     O                              
450       Δ                                                         
                O      O   O    O    O                                    
500       Δ                                                         
                O      O   O    O                                         
550       Δ                                                         
                O      O   O                                              
600       Δ                                                         
                O      O   O                                              
650       Δ                                                         
                O      O   O                                              
700       Δ                                                         
                O      O                                                  
750       Δ                                                         
                O      O                                                  
800       Δ                                                         
                O      O                                                  
850       Δ                                                         
                O      O                                                  
900       Δ                                                         
                O      O                                                  
950       Δ                                                         
                X      X                                                  
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     Further Samples 4, 16, 23, 24 and 31 were immersed in a (Na 4 ) 2  SX) for 30 min. Discoloration into brown tint started at a period of 1 min. from beginning of the immersion and dark blue tint was reached at the period of 30 min. During the test, the samples exhibited elegant color suited for decorative purposes. After the immersion, the samples were left in the atmospheric environment for 6 months, but no substantial change in color was observed while maintaining the initial elegant tint.